1. Field of the Invention
The present invention relates to a carbonaceous protective layer, a magnetic recording medium used in a hard disk device of a computer, their production methods, and a magnetic disk device or apparatus using the magnetic recording medium. More particularly, the present invention relates to a carbonaceous protective layer for protecting a magnetic recording layer of a magnetic recording medium, etc, and a production method of the carbonaceous protective layer.
2. Description of the Related Art
In an information processing unit such as a computer, a magnetic disk apparatus has been widely used as an external storage device. When the magnetic disk apparatus is used, information can be recorded on and read from the magnetic recording medium as a magnetic head scans the magnetic recording medium (magnetic disk). Various improvements have been made in both the magnetic recording medium and the magnetic head in order to satisfy recent high-level needs such as high-density recording and recording and reproduction with high sensitivity and at a high speed.
As is well-known, a conventional magnetic recording medium comprises a non-magnetic substrate having applied thereon, in sequence, an underlayer, a magnetic recording layer (also called as a “magnetic layer”), a protective layer and a lubricant layer. The substrate comprises an aluminum substrate, for example, which has a NiP-plated surface. This surface is super-finished. Super-finishing smoothes the surface of the substrate. The underlayer is generally made of a Cr-based alloy as a non-magnetic metal. The Cr-based alloy is a CrMo-based alloy, for example. The magnetic recording layer is generally made of a CoCr-based alloy as a ferromagnetic metal. The CoCr-based alloy is CoCrTa, CoCrPt or CoCrPtTaNb, for example. The protective layer is deposited to the magnetic recording layer to protect the magnetic recording layer from damage resulting from impact with the magnetic head. The protective layer is made of various carbon materials such as amorphous carbon. The protective layer is generally called a “carbonaceous protective layer”. The carbonaceous protective layer is impregnated with a liquid lubricant such as a fluorocarbon-based liquid lubricant to form the lubricant layer that insures smooth flying of the head above the magnetic recording medium.
In the magnetic recording medium according to the prior art, the carbonaceous protective layer has been formed by sputtering, chemical vapor deposition (hereinafter referred to as “CVD”), etc, that are conventional film-forming technologies in the production of semiconductor devices. To impart improved durability to the carbonaceous protective layer so formed, hydrogen and nitrogen are often added to the carbonaceous protective layer. For example, Japanese Unexamined Patent Publication (Kokai) No. 7-296372 discloses a magnetic recording medium formed by serially laminating a magnetic layer, a carbonaceous protective layer and a lubricant layer on a non-magnetic substrate. In this magnetic recording medium, the surface of the carbonaceous protective layer is plasma-treated in an ammonia gas-containing atmosphere and then a lubricant layer is formed by using a lubricant containing a lubricant molecule having a carboxyl group at one of the terminals. In this magnetic recording medium, the carbonaceous protective layer is a carbon layer or a hydrogenated carbon layer, and is formed by sputtering, plasma CVD or ion plating. The thickness of such a carbonaceous protective layer is generally 50 to 500 angstroms and preferably 100 to 300 angstroms.
A similar magnetic recording medium is also disclosed in Japanese Unexamined Patent Publication (Kokai) No. 10-143836. The magnetic recording medium described in this reference includes a ferromagnetic metal thin film formed on a non-magnetic substrate and a protective layer formed on the ferromagnetic metal thin film. The protective layer is a nitrogen-containing carbonaceous layer characterized in that a nitrogen concentration in the protective layer is varied in the thickness-wise direction of the protective layer, a nitrogen concentration of the layer on the substrate side is higher than that of the layer on the surface side, and a lubricant layer on the protective layer contains a polyphenoxycyclotriphosphazene lubricant in a weight ratio of 0.01 to 1.0 in addition to perfluoropolyether lubricant.
Though hydrogen and nitrogen are added to the carbonaceous protective layer of the conventional magnetic recording media to improve durability, these media cannot exhibit sufficiently high durability when the thickness is reduced in the protective layer. That is, although the hard disk apparatuses have rapidly become to have higher recording density, and thus the flying height of the head as well as the film thickness of the protective layer have been reduced, the carbonaceous protective layer, when formed with a small thickness, cannot is still insufficient to provide an improved durability. As a matter of fact, even when nitrogen is added to the carbonaceous protective layer formed by sputtering or CVD, so as to improve its durability, this durability can hardly be maintained in the case of a thin film having a layer thickness of 5 nm or less.